Prior to the present invention, the plastics industry expended considerable effort towards the study and development of high performance thermoplastic foams for the purpose of making lightweight structural substitutes for metal in various applications such as used in the automotive industry. Examples of these blowing agents include hydrazodicarboxylates and the amide derivatives of azodicarboxylic acid, such as azodicarbonamide. As shown by Hunter et al, U.S. Pat. No. 3,888,801, hydrazodicarboxylates have commonly been employed as blowing agents for various thermoplastic organic polymers for the purpose of reducing the overall weight of particular thermoplastic materials when molded to a specific shape. Other blowing agents, which are commonly used with various thermoplastic organic polymers, are 5-phenyl-tetrazole, benzamides, etc., as shown by Muller et al, U.S. Pat. No. 3,781,233, and Wirth et al, U.S. Pat. No. 3,779,954. Although hydrazodicarboxylates, such as diisopropyl hydrazodicarboxylate and 5-phenyltetrazole have generally been found effective for reducing the density of many thermoplastic organic polymers, such as polycarbonates, polyesters, polyacrylates, etc., it has been shown that a significant degree of polymer degradation can occur during the foaming process. Polymer degradation is generally evidenced by a reduction in the intrinsic viscosity of the polymer when a comparison of polymer intrinsic viscosity is made before and after foaming. Polymer degradation is also directly related to reduction in foam impact strength.
Although it is not completely understood, one possible explanation as to why blowing agents, such as the above-described hydrazodicarboxylates can cause a significant degree of polymer degradation upon foaming, is that such blowing agents have decomposition byproducts such as aliphatic alcohols, ammonia, water, etc.
Blowing agents such as the above-described benzazimides and bisbenzazimides are also polymer degradation prone, since water is a decomposition byproduct. Those skilled in the art know that careful drying of high performance thermoplastics, such as polycarbonates, is required prior to molding because of possible risk of polymer degradation. It would be desirable, therefore, to provide blowing agents which can be employed in a variety of high performance thermoplastic organic polymers, such as polyesters, polycarbonates, etc., which do not degrade the thermoplastic polymer and which resist an adverse reduction in impact strength of the foam product beyond that normally expected from the change in the density of the material as the result of foaming.
Such blowing agents are the dihydrooxadiazinones described in U.S. Pat. No. 4,097,425 to Niznik. While these blowing agents generally are satisfactory and avoid the disadvantages of the prior mentioned blowing agents, it has now been found that, if a second blowing agent which is an amide derivative of azodicarboxylic acid is added to the blends containing said dihydrooxadiazinones, the density of the foamed structures obtained from these foamable thermoplastic compositions is reduced. This results in foamed structures having less stressing and warpage than foamed structures having higher densities. Furthermore, the cellular structure of the foamed structures obtained from these blends is well defined and uniform.